Counersinks in Thin Plastic

Atomkinder · Feb 10, 2016

Countersinks in Thin Plastic

We're having an issue with running a countersink in a relatively thin plastic disc.

The part has a through-hole and we're using a 3/8" MA Ford single flute 100° countersink but getting inconsistent results even with a dwell. I suspect the single flute is causing oscillation in the surface (there's a clearance channel in the fixture under the part) giving us poor consistency. The disc is about .116" thick and the countersink varies from .13" depth to .144". Part of the issue might also be the diameter of the cutter, but I wonder if a zero flute or multi-flute tool would be better in order to keep pressure on the part to keep it from rising in the cut.

Any similar experiences are welcome.

Current parameters are 1222 RPM at 7.334 IPM, started with an air blast, and removing it made no difference (was curious if the air was blowing the part up off the fixture slightly). Plastic material seems very similar to Delrin.

I might be able to take a few photos if anyone things it would help.

specfab · Feb 10, 2016

Random notes:
You can try reducing feed rate so as to not depress the material as much. If you have a void under the part, and seemingly non-symmetrical relative to countersink, that is bad news for countersink consistency. How big is the disk, and how are you holding it?

Perhaps a zero-rake cutting edge (scrape, not dig in) might be worth trying, with a reduced feed rate.

The Weldon Zero-flute type cutter may also be very viable, but I would bet that the spindle feed pressure and the under-part support are the primary issues.

Mike Ortega · Feb 10, 2016

Is there a inconsistencies between parts? (thickness, bowing) Can you clamp closer to the hole and use a chamfer mill?

2outof3 · Feb 10, 2016

You could try freezing the parts first.

david n · Feb 10, 2016

Milling or turning?

If milling, interpolate it with a 100° spot drill or mill drill........................

Monster Tool N/C Spotting Drills 100° Included Angle

Melin Drill Point End Mills

BALNH · Feb 10, 2016

Interpolate or 3d mill with a ball mill, start from center and spiral out. I do it on prototypes for c'sinks that I don't have on hand.

allloutmx · Feb 10, 2016

I agree with trying it with a bull or ball EM, but you might still have problems with it pulling if you start from the bottom, so I might try starting from the top first

Atomkinder · Feb 10, 2016

specfab said:
Random notes:
You can try reducing feed rate so as to not depress the material as much. If you have a void under the part, and seemingly non-symmetrical relative to countersink, that is bad news for countersink consistency. How big is the disk, and how are you holding it?

Perhaps a zero-rake cutting edge (scrape, not dig in) might be worth trying, with a reduced feed rate.

The Weldon Zero-flute type cutter may also be very viable, but I would bet that the spindle feed pressure and the under-part support are the primary issues.

I will reduce the pressure first and see if that helps, as I was thinking in a more traditional setup that a higher chipload works better in plastics. I can see how that might not be the case in this situation.

Right now there's a void under the part, but it is supported on the outside (there's a slot under the diameter that gets drilled and countersunk). Part is held on location and by a large domed plate that puts pressure close to the "bolt circle", and the blank is about 12" in diameter.

Mike Ortega said:
Is there a inconsistencies between parts? (thickness, bowing) Can you clamp closer to the hole and use a chamfer mill?

Part blanks are very consistent, and bow upwards if at all, but are held down pretty close to the drill diameter (think a bolt circle).

2outof3 said:
You could try freezing the parts first.

While that would work right now just putting it outside (I think the current temperature is about 15°F) that's not a viable option year-round. We do about 20 variations of this part and are now correcting all our old programs that use a 90° tool (which was previously a center drill).

david n said:
Milling or turning?

If milling, interpolate it with a 100° spot drill or mill drill........................

Monster Tool N/C Spotting Drills 100° Included Angle

Melin Drill Point End Mills

That's going to take too long. Some of these parts have 60 holes in them.

BALNH said:
Interpolate or 3d mill with a ball mill, start from center and spiral out. I do it on prototypes for c'sinks that I don't have on hand.

allloutmx said:
I agree with trying it with a bull or ball EM, but you might still have problems with it pulling if you start from the bottom, so I might try starting from the top first

Way too time intensive. Countersinks are cheap and these are recurring production parts.

Whysosharp · Feb 10, 2016

If the material is nylon then that would explain its inconsistency as it tends to wobble about delrin at that thickness is 3x more rigid and wouldn't be so inconsistent in my experience. Your best bet and it may seem drastic would be to put some double sided tape down and sick your part down with it. That'll help it from pulling and bending so much. If your not using coolant then taping it won't be a problem. Make sure that countersink is razor sharp maybe even try to get a veining bit with a 100deg tip if you can find one.

Atomkinder · Feb 10, 2016

Whysosharp said:
If the material is nylon then that would explain its inconsistency as it tends to wobble about delrin at that thickness is 3x more rigid and wouldn't be so inconsistent in my experience. Your best bet and it may seem drastic would be to put some double sided tape down and sick your part down with it. That'll help it from pulling and bending so much. If your not using coolant then taping it won't be a problem. Make sure that countersink is razor sharp maybe even try to get a veining bit with a 100deg tip if you can find one.

It's much more Delrin-like. I will see if I can find out what it is, but it's an injection-molded blank that we buy from a supplier.

Really, really don't want to tape it down. We do anywhere from one to 30 of these at at a time, both sides. The fixture has worked well generally, and this is the first issue I've seen, but now we're using new tooling.

Limy Sami · Feb 10, 2016

From hard won experience - I had 000's upon 000's to do in a DP :eek:

I use a TG single flute countersink /chamfer mill like this 1 Flute 90° HSS 8% Cobalt Chamfer Mill (C1119 Series) -you should be able to find the angle you want from US tool firms.

Dub the edge back with an oilstone as you would a drill for brass, and that edge has to be SHARP, as in 10X glass and white Arkansas stone SHARP and the tool NOT used for anything else!

A simple D bit countersink works well, but I can only make them out of drill rod, and it wouldn't hold u[p at the speeds I wanted to run on that long job.

Above all that tool has to be SHARP!

Spyderedge · Feb 10, 2016

Take a countersink, and hard turn or grind it to the same diameter as the largest diameter of the chamfer, then make a stop collar that sits on the tool. Limiting chamfer size and depth. Might work?

Limy Sami · Feb 10, 2016

Spyderedge said:
Take a countersink, and hard turn or grind it to the same diameter as the largest diameter of the chamfer, then make a stop collar that sits on the tool. Limiting chamfer size and depth. Might work?

Yes but it will mark the plastic.

Spyderedge · Feb 10, 2016

Limy Sami said:
Yes but it will mark the plastic.

What about a delrin or nylon collar? Nylon and its self lubricating properties might help prevent marking, as long as its clean.

Atomkinder · Feb 10, 2016

Spyderedge said:
What about a delrin or nylon collar? Nylon and its self lubricating properties might help prevent marking, as long as its clean.

I don't know if I trust the machine to stop on that stop. The fixture has support on the outside of the part, just has a relief slot under the "bolt circle" area.

2outof3 · Feb 10, 2016

Atomkinder said:
While that would work right now just putting it outside (I think the current temperature is about 15°F) that's not a viable option year-round. We do about 20 variations of this part and are now correcting all our old programs that use a 90° tool (which was previously a center drill).

.

I am thinking really freezing. Like drop them in liquid nitrogen for about a minute before clamping. I have had customers who machine rubber where all material is kept in a walk in freezer. You could just throw the blanks in the freezer in the lunch room overnight before cutting them.

Atomkinder · Feb 10, 2016

2outof3 said:
I am thinking really freezing. Like drop them in liquid nitrogen for about a minute before clamping. I have had customers who machine rubber where all material is kept in a walk in freezer. You could just throw the blanks in the freezer in the lunch room overnight before cutting them.

Not gonna happen.

This is far more "new tooling issues" than anything else.

For now we've ordered a 1/2" 3-flute 100° countersink and we'll see how that goes. It's a cheap experiment.

Dupa3872 · Feb 10, 2016

I didn't read all the posts. Try circle milling these. Check out Harvey Tool and see if they have a cutter with the angle you require then just drop into the center and away you go. If the stock is down flat and the thickness is. Consistent you will do just fine.

Good luck

Make Chips Boys !

Ron

CarbideBob · Feb 10, 2016

You are running catalog steel cutting c-sinks on thin plastic parts ?
Yea, that's not gonna work so well.
Bob

adama · Feb 10, 2016

You did not hear it from me, but a humble twist drill ground at the right angle works too. A protractor + well dressed bench grinder :-) Would go a lot slower on speed too no support plastics get hot and move all the faster in thin sections!

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